This invention relates to electrosurgery and is more particularly concerned with electrosurgical coagulation.
The purpose of electrosurgery is to provide both coagulation and cutting of living tissue during surgical procedures. Coagulation is the stoppage of blood and other fluids flowing from or through the tissue and is accomplished in electrosurgery by the dehydration of cells. Cutting is the surgical incision or excision of tissue and is accomplished in electrosurgery by the volatilization of cells. Hemostasis is a blend of both cut and coagulate.
These modes are very much functions of the peak and average power applied to the tissue. Let us consider an electrosurgical unit having an RF generator, the output of which is pulse modulated so as to provide an on-time of RF energy followed by an off-time. The RF energy is applied to the patient's tissue by means of electrodes. During the on-time the tissue is heated by the RF energy, while during the off-time the tissue cools.
For periodic modulation, duty cycle may be defined as the ratio of pulse on-time to the sum of pulse on and off time.
It is recognized that, for signals having a constant peak voltage, the energy transferred by the modulated RF signal is directly proportional to the duty cycle. Therefore, it has been necessary to provide higher voltage during coagulation than during cut so that high average power may be maintained for efficient coagulation while still allowing a cool off time sufficient to prevent cutting. Accordingly, the preferred conditions for electrosurgical coagulation has been an RF signal characterized by low duty cycle and high voltage.
An early electrical surgical device is the spark gap generator. This device has a peak voltage of up to 10,000 volts and a duty cycle of perhaps four percent. (The duty cycle of a spark gap can only be estimated, as the time between spark gap pulses may vary). The envelope of RF energy produced by the spark gap has been described as a damped sinusoid. The spark gap generator has enjoyed success for over 60 years, but due to maintenance, reliability, electrical interference, and other considerations it is becoming replaced by electronic electrosurgical units, and in particular, by solid state transistorized units. Transistorized units generally do not provide as high a voltage as does a spark gap generator and so the duty cycles of transistorized units are increased somewhat to provide a high power. This technique is limited as to how high the duty cycle can be increased because, even with lower voltage, too high a duty cycle will cause unwanted cutting.
It has been found that a duty cycle of 15% to 20% is usually suitable for coagulation. The relatively long offtime allows tissue to cool before volatilization can occur. A duty cycle of 60% or more is preferred for cutting. The high average power heats the tissue sufficiently to volatilize it. Duty cycles of about 50% to 60% are useful for hemostasis.